The lateral line system is a mechanosensory hair cell system found in nearly all aquatic vertebrates. Like inner ear hair cell systems, the lateral line transduces mechanical stimulation. Rather than sensing postural changes or sound, however, the lateral line senses water motions that accompany acoustic stimuli in fluids. Yet the principles of stimulus encoding used by the two systems may be quite similar. This proposal develops a set of hypotheses that relate source localization to the spatial pattern of lateral line stimulation in much the same way that frequency discrimination is related to the pattern of stimulation in the cochlea. The specific aims of this proposal test these hypotheses by examining how the distance range and spatial discriminatory ability of the lateral line system is determined by the spatial distribution of sensory organs. Differences in lateral line structure are presented as they would affect the spatial pattern of lateral line excitation and thus provide a test of the excitation pattern model of lateral line source localization. This research promises to increase our understanding of how such spatial patterns of excitation along hair cell arrays are used by the nervous system to extract information about stimulus sources.